Sensors and Actuators B: Chemical
Sensitivity enhancement in a multimode interference-based SMS fibre structure coated with a thin-film: Theoretical and experimental study


Multimode interference in a single-mode–multimode–single-mode (SMS) fibre structure leads to the generation of transmission bands and attenuation bands in the optical spectrum. In this work, the progressive deposition, with layer-by-layer assembly technique, of a high refractive index thin-film onto the SMS structure permits to observe the evolution of the bands as a function of the coating thickness. The results show that it is possible to increase the sensitivity of the device to variations of the coating thickness, what indicates the adequateness of this structure as a sensing platform. The experimental results are corroborated with a numerical method based on coupled-mode theory. As a practical application, a sensitivity improvement has been obtained a pH sensor by increasing the coating thickness.


In view of the wavelength range monitored with the set-up shown in Fig. 1 a good choice is to set the self-image transmission band in one of the optical communications window. According to Section 2.1 and considering that the diameter and the refractive index of the MMF section are 125 μm and 1.444 respectively, the self-image transmission band can be located at 1550 nm (the centre of the third optical communications window) by using a 58 mm-length MMF segment. That is why a 58 mm-length coreless MMF segment from POFC Inc. was spliced into two standard SMF pigtails from Telnet Inc. The external diameter of the SMS structure was 125 μm, as it is also depicted in Fig. 1.

Once the optical substrate was obtained, it was fixed to a deposition robot (ND-XY, designed by Nadetech Innovations S.L.). In this work, a thin-film was adsorbed onto the SMS structure by using the layer-by-layer assembly technique (LbL) [23]. The process consisted in the alternate immersion of the substrate into two solutions of poly (allylamine hydrochloride) (PAH) and poly (acrylic acid) (PAA), both of them in 10 mM concentration and adjusted to pH 4.5. Chemicals were purchased from Sigma-Aldrich. The environmental conditions were maintained at 23 °C of temperature and 18% of relative humidity.


Keywords: Single-mode–multimode–single-mode (SMS); Fibre structure; Layer-by-layer; Thin-films; Optical fibre sensors